The proposed experiments will address the fundamental question of how rostrocaudal differences arise in neural crest populations. Our premise is that there may be inherent molecular differences between cranial and trunk neural crest cells that lead to differences in migratory behavior and cell fate. Although both can contribute to elements of the peripheral nervous system, only cranial but not trunk neural crest cells are able to contribute to osteogenic and chondrogenic lineages. We will test our hypothesis by characterizing the function of genes that are differentially expressed between cranial and trunk neural crest populations. Using differential display, we have identified a number of candidate genes that are differentially expressed in cranial or trunk neural fold/tube populations. The proposed experiments will test the mechanistic role of some of these genes in contributing to the differences in developmental potential between cranial and trunk neural crest populations. First, we will explore the possible function of a helix-loop-helix transcriptional regulator, chick Id2, that is selectively expressed in the cranial but not the trunk neural folds. We will test whether this molecule plays a role in proliferation and/or differentiation of cranial neural crest population by over- expression and transplantation; we will test the effects of its mis-expression in the trunk to see if it causes malformations such as ectopic cartilage formation. Second, we will characterize the role of an E protein, with which Id2 may interact, in craniofacial development. Preliminary experiments reveal that E12 is expressed in the cranial neural folds in a pattern that overlaps with Id2. Finally, we will isolate a chick achaete-scute homolog based on the observation that, in frogs, Xash-3 has the proper distribution pattern to interact with Ids. Our working hypothesis is that interactions between Id2/E12/Cash are important for proper craniofacial patterning. Specific experiments will: 1. analyze the role of Id2 in a cranial neural crest cell differentiation in the chick. 2. examine the function of E12 in neural crest development and craniofacial patterning. 3. identify and characterize an achaete-scute homolog(s) that potentially interacts with Id2/E proteins.